Solid state light sources are currently entering many different lighting applications and replace incandescent and gas discharge lamps. Several applications are characterized by a small Étendue of the optical system, e. g. digital protection, optical fibres, architectural, entertainment or stage lighting. High intensity gas discharge lamps, like for example UHP- or Xenon-lamps are currently used in these demanding applications. Nevertheless, solid state light sources are also highly desired as the light source in this type of applications. LEDs are still limited in their brightness and therefore can not be adapted to low Étendue applications. Lasers provide a brightness that is by orders of magnitude higher than the typical requirements of optical applications with low Étendue. This means that there are basically no collection losses to be expected, which makes lasers a very attractive light source for such applications. However, suitable lasers are not available at all required emission wavelengths. Especially in the green wavelength range there is a lack of direct laser diodes with appropriate performance and costs. Furthermore, laser projection is still hampered by Speckle and other interference effects which require additional measures to ensure an acceptable picture quality. These effects are related to the small bandwidth (or high coherence) of lasers.
A new type of light source for projection was recently proposed in which a laser excites a phosphor layer that is placed on a rotating disc. This allows the design of a hybrid RGB-light source, where the blue light is generated by an array of laser diodes, the green light is generated by using—in a time sequential manner—a portion of the blue light to excite the phosphor and convert from the blue to the green, and the red light is generated with a red emitting LED. To keep the temperature of the phosphor in an appropriate range where the conversion efficiency is still good, the phosphor is placed on a rotating wheel. In this way the average laser power that hits the phosphor is spread out and kept low enough so that the temperature of the phosphor remains within an uncritical range. This type of light source makes good use of the laser properties, since it is possible to focus laser beams to very tiny spot sizes and excite only a small area on the phosphor. The smaller the spot size, the smaller is the optical Étendue into which the light is emitted and in this way, a low Étendue light-source is realized. Depending on the type of phosphor, this light-source emits a relatively broad spectrum and does not cause any disturbing Speckle-effects.
US 20110116253 A1 discloses such a solid state light source which includes a phosphor covered disc and a motor rotating the disc in the beam path of a semiconductor laser diode. The disc is made from an optically transparent material such as quartz glass and rotates around a central rotational axis. An optical lens system is arranged behind the rotating disc in order to collect the converted radiation emitted by the phosphor layer.